1. Field of the Invention
The present invention relates to driving-force transmitting apparatuses for four-wheel-drive vehicles capable of switching between two-wheel drive and four-wheel drive and, in particular, a driving-force transmitting apparatus for a four-wheel-drive vehicle stopping the rotation of a portion not related to transmission of driving force in two-wheel drive.
2. Description of the Related Arts
In a conventional so-called on-demand-type full-time four-wheel-drive vehicle, as an example of a driving-force transmitting apparatus for an FF-vehicle-base four-wheel-drive vehicle that uses an electronic control coupling device to control distribution of a driving force in a manner such that front wheels are driven in a two-wheel-drive mode and rear wheels are driven in a four-wheel-drive mode, an apparatus shown in FIG. 1 is known.
In FIG. 1, a driving-force transmitting apparatus 610 is provided in a four-wheel-drive vehicle 612, and the speed of a driving force from an engine 628 is changed by a change gear 630 for input to a front-wheel differential device 614 from a drive gear 632 of the change gear 630 via a ring gear 634 and also transmission to an output pinion 664 via a bevel gear 662 integrally rotating with the ring gear 634 connected to the front-wheel differential device 614. A driving force from the output pinion 664 is transmitted to an electronic-control coupling device 624 having a multi-plate clutch mechanism 672 and an actuator 674 via a propeller shaft 668. In two-wheel drive, when the electronic-control coupling device 624 is cut off (in the state where the multi-plate clutch mechanism 672 is released), the driving force is not transmitted to a rear-wheel differential device 618 but is transmitted only to the front-wheel differential device 614. While absorbing a difference in rotation speed between a left-front wheel 650 and a right-front wheel 652, the front-wheel differential device 614 provides the same torque to the left-front wheel 650 and the right-front wheel 652 for rotation. In four-wheel drive, when the electronic-control coupling device 624 is connected in four-wheel drive (in the state where the multi-plate clutch mechanism 672 is connected), the driving force is transmitted also to the rear-wheel differential device 618 from a drive pinion 676 coupled to the electronic-control coupling device 624 via a ring gear 678. While absorbing a difference in rotation speed between a left-rear wheel 700 and a right-rear wheel 702, the rear-wheel differential device 618 provides the same torque to the left-rear wheel 700 and the right-rear wheel 702. The electronic-control coupling device 624 operates the actuator 674 with a control signal E6 from an ECU 626 to successively change a connection force of the multi-plate clutch mechanism 672, thereby allowing control of the torque to be transmitted via the rear-wheel differential device 618 to the left-rear wheel 700 and the right-rear wheel 702 between zero and a predetermined maximum torque. With this, in the two-wheel drive mode, the transmission torque of the electronic-control coupling device 624 is controlled at zero to cause the driving force to be transmitted only to the front-wheel differential device 614. In the four-wheel drive mode, the transmission torque of the electronic-control coupling device 624 is controlled at an appropriate torque according to the running state of the vehicle for transmission also to the rear-wheel differential device 618. Also, as an example of a driving-force transmitting apparatus for an FR-vehicle-base four-wheel-drive vehicle, the apparatus driving the rear wheels in the two-wheel drive mode and controlling distribution of the driving force to the front wheels with an electronic-control coupling device in the four-wheel drive mode, an apparatus shown in FIG. 2 is known.
In FIG. 2, a driving-force transmitting device 710 is provided in a four-wheel-drive vehicle 712, and the speed of a driving force from an engine 728 is changed by a change gear 730 for input to an electronic-control coupling device 724 having a multi-plate clutch mechanism 772 and an actuator 774. In two-wheel drive, when the electronic-control coupling device 724 is cut off (in the state where the multi-plate clutch mechanism 772 is released), the driving force is directly transmitted to a rear-wheel differential device 714 via a propeller shaft 762, a drive pinion 732, and a ring gear 734. While absorbing a difference in rotation speed between a left-rear wheel 750 and a right-rear wheel 752, the rear-wheel differential device 714 provides the same torque to the left-rear wheel 750 and the right-rear wheel 752 for rotation. In four-wheel drive, when the electronic-control coupling device 724 is connected in four-wheel drive (in the state where the multi-plate clutch mechanism 772 is connected), the driving force is transmitted also to a front-wheel differential device 718 from a chain belt mechanism 764 coupled to the multi-plate clutch mechanism 772 via a propeller shaft 768, a drive pinion 776, and a ring gear 778 on a front-wheel side. While absorbing a difference in rotation speed between a left-front wheel 800 and a right-front wheel 802, the front-wheel differential device 718 provides the same torque to the left-front wheel 800 and the right-front wheel 802. The electronic-control coupling device 724 operates the actuator 774 with a control signal E7 from an ECU 726 to successively change a connection force of the multi-plate clutch mechanism 772, thereby allowing control of the torque to be transmitted via the front-wheel differential device 718 to the left-front wheel 800 and the right-front wheel 802 between zero and a predetermined maximum torque.
With this, in the two-wheel drive mode, the transmission torque of the electronic-control coupling device 724 is controlled at zero to cause the driving force to be transmitted only to the rear-wheel differential device 714. In the four-wheel drive mode, the transmission torque of the electronic-control coupling device 724 is controlled at an appropriate torque according to the running state of the vehicle for transmission also to the front-wheel differential device 718.
However, in the conventional driving-force transmitting apparatus 610 as depicted in FIG. 1, even in the two-wheel drive mode, the driving force from the change gear 630 rotates a driving side (front-wheel side) of a bevel gear 662, the output pinion 664, the propeller shaft 668, and the multi-plate clutch mechanism 672. Also, with the left-rear wheel 700 and the right rear wheel 702 and the rear-wheel differential device 618 being directly connected, the left-rear wheel 700 and the right-rear wheel 702 rotate with the driving force from the road surface, thereby causing a driven side (rear-wheel side) of the ring gear 678, the drive pinion 676, and the multi-plate clutch mechanism 672 to rotate. That is, in the two-wheel drive mode in which the electronic-control coupling device 624 is cut off and the driving force is not transmitted to the rear wheels, even if the multi-plate clutch mechanism 672 is completely released, each component of a driving-force transmitting unit 616 including the bevel gear 662, the output pinion 664, the propeller shaft 668, the multi-plate clutch mechanism 672, the drive pinion 676, and the ring gear 678 rotate via the rear-wheel differential device 618. Moreover, in the conventional driving-force transmitting device 710 as depicted in FIG. 2, even in the two-wheel drive mode in which the multi-plate clutch mechanism 772 is released and the driving force is not transmitted to the front wheels, since the left-front wheel 800 and the right-front wheel 802 and the front-wheel differential device 718 are directly connected, the left-front wheel 800 and the right-front wheel 802 rotate with the driving force from the road surface, thereby causing each component of the driving-force transmitting unit 716 including the ring gear 778, the drive pinion 776, the propeller shaft 668 on a front-wheel side, and the chain belt mechanism 764 to rotate via the front-wheel differential device 718. In this manner, in the FF-vehicle-base four-wheel-drive vehicle 612 depicted in FIG. 1, although two-wheel drive can be achieved, in terms of driving force, by setting the transmission torque of the electronic-control coupling device 624 at zero, the driving-force transmitting unit 616 always rotates as transmitting the driving force from the engine to the front-wheel-differential device 614 and to the rear wheels. Furthermore, also in the FR-vehicle-base four-wheel-drive vehicle 712 depicted in FIG. 2, as with the case of the FF-vehicle base of FIG. 1, although two-wheel drive can be achieved, in terms of driving force, by setting the transmission torque of the electronic-control coupling device 724 at zero, the driving-force transmitting unit 716 always rotates as transmitting the driving force from the engine to the electronic-control coupling device 724 to front wheels. For this reason, in these conventional four-wheel-drive vehicles 612 and 712, fuel efficiency is disadvantageously deteriorated even in the two-wheel drive mode, compared with that of a two-wheel-drive vehicle, due to agitation resistance of oil, friction loss of a bearing portion, and other factors in the driving-force transmitting units 616 and 716.